Conventionally, for window glass of vehicles such as automobiles, glass articles are used such as anti-fogging glass provided with anti-fogging hot wires and glass antennas for receiving ratio waves from the outside of vehicles. These glass articles typically have a conductive film formed in a predetermined pattern by applying a conductive paste in the form of lines onto a glass substrate and firing the paste. This type of conductive paste for glass articles has been researched and developed actively.
For example, Japanese Patent No. 3767514 (Patent Document 1) proposes therein a conductive paste including an Ag powder, glass frit, and an organic vehicle, for anti-fogging hot wires of automotive windows, where a silver powder of 4.5 μm to 7 μm in particle size and 100 nm or more in crystallite size accounts for 50 wt % or more of the Ag powder. According to this patent, the use of the conductive paste in which 50 wt % or more of the Ag powder has predetermined particle sizes and crystallite sizes retards sintering of the Ag particles, thereby attempting to achieve anti-fogging hot wires which have favorable solder wettability, keep the joint strength from being decreased, and have a high resistivity suitable for automotive window glass.
In addition, Japanese Patent No. 4805621 (Patent Document 2) proposes a conductive paste including a first Ag powder of 5 μm or less in average particle size, obtained by an atomization preparation method and a second Ag powder within the range of 0.2 to 2.0 μm in average particle size, obtained by a wet reduction method, in proportions by mass within the range of 20/80≤(first Ag powder/second Ag powder)≤80/20, where the second Ag powder includes a fine powder within the range of 1.0 to 2.0 μm in average particle size, and an ultra-fine powder within the range of 0.2 to 0.6 μm in average particle size. According to this patent, the blend of the atomized powder having the predetermined particle sizes and the wet reduced powder having the predetermined particle sizes in the predetermined proportions by mass attempts to achieve a conductive paste which suppresses firing shrinkage, and has a low resistance value after firing.
Conventional methods for preparing conductive powders such as Ag powders used in conductive pastes include wet reduction methods and atomization methods. Further, Ag powders with relatively large particle sizes on the order of 4.5 μm to 7 μm as in Patent Document 1 are preferably prepared by the atomization methods rather than the wet reduction methods, in consideration of cost.
However, when the Ag powder according to Patent Document 1 is prepared by an atomization method, and when a conductive paste containing the atomized powder is applied in the form of a line onto a glass substrate and subjected to firing, there is a possibility that the glass substrate around the fired conductive film will undergo a color change to a yellow color or the like. When such conductive pastes are used in automotive window glass, there is a possibility that the visibility of the drivers will be adversely affected. On the other hand, when a wet reduced powder is used, there is a possibility that an increase in cost will be caused as compared with atomized powders as described above, although there is no color change to a yellow color or the like.
In addition, as automotive window glass, it is common to form, on a glass substrate, a blackish ceramic layer that has an antidazzle function, and form a conductive film on the ceramic layer in the form of a line. In this case, for the automotive window glass mentioned above, typically, a ceramic dried film is formed on the glass substrate, thereafter, a conductive paste is applied onto the ceramic dried film, and dried, and the ceramic dried film and the conductive paste are subjected to co-firing, thereby forming the ceramic layer and the conductive film.
However, while Patent Document 2 makes an attempt to suppress firing shrinkage by adjusting the combination ratio between the atomized powder of 5 μm or less in average particle size and the wet reduced powder of 0.2 to 2.0 μm in average particle size, the average particle size of 5 μm or less decreases the effect of suppressing firing shrinkage even when the low-sinterability atomized powder is used, and for this reason, ceramic layers have the possibility of having structural defects such as cracks generated after firing.
It is an object of the present invention to provide a conductive paste which can prevent a glass substrate from undergoing a color change and prevent a base layer for a conductive film from having structural defects such as cracks generated. It is also an object of the present invention to provide a glass article, such as anti-fogging glass, using the conductive paste.
For atomized powders, jet flows are sprayed to molten metals to make droplets, and the droplets are solidified, thereby providing metal powders. Therefore, as compared with wet reduced powders obtained in a way that aqueous solutions of metal salts are reduced to deposit metal powders, the surfaces can be prevented from having unevenness, and the specific surface areas can be thus reduced, thereby decreasing the sinterability.
On the other hand, as mentioned in the section of [Problem to be solved by the invention], the use of only the atomized powder as the conductive powder unfavorably causes the glass substrate near the conductive film to undergo a color change after firing.
Therefore, the inventors have earnestly carried out studies with the use of mixed powders of an atomized powder and a conductive powder other than the atomized powder, for example, a wet reduced powder, thereby finding that because of minute amounts of chlorine components included in molten metal and spray water in the process of producing the atomized powder, the chlorine components are mixed as impurities into the conductive powder, and for this reason, the glass substrate near the conductive film undergoes a color change after firing.
Further, the inventors have earnestly carried out further studies, thereby providing the finding that as long as the content of the chlorine component mixed in the conductive powder is 42 ppm or less, the conductive powder contains therein a predetermined amount of atomized powder with an average particle size adjusted within a predetermined range, thereby making it possible to prevent the glass substrate from undergoing a color change, and a base layer for the conductive film from having structural defects such as cracks generated.
The present invention has been made on the basis of the foregoing finding, and a conductive paste according to the present invention is a conductive paste containing at least a conductive powder, glass frit, and an organic vehicle, in which the conductive powder contains an atomized powder in a range of 5 to 40 wt %, the atomized powder is 5.2 to 9 μm in average particle size, and a content of a chlorine component mixed in the conductive powder is 42 ppm or less.
It is to be noted that average particle size mentioned above is referred to as a particle size corresponding to 50% of the cumulative distribution, that is, a median size (hereinafter, an “average particle size D50”) in the present invention.
In addition, in the conductive paste according to the present invention, the conductive powder preferably contains a wet reduced powder.
The mixed powder of the atomized powder and the wet reduced powder is adopted for the conductive powder as just described, thereby making it possible to achieve, in an effective manner, a conductive paste which undergoes no color change to a yellow color or the like after firing.
In addition, in the conductive paste according to the present invention, a content of the conductive powder is preferably 50 to 90 wt %.
Furthermore, in the conductive paste according to the present invention, an average particle size of the glass frit is preferably twice or less as large as an average particle size of the conductive powder.
In addition, in the conductive paste according to the present invention, the glass frit preferably has a softening point of 350° C. to 600° C.
In addition, in the conductive paste according to the present invention, the conductive powder preferably contains Ag as a main constituent.
In addition, a glass article according to the present invention is a glass article including a conductive film in a predetermined pattern formed on a glass substrate, in which the conductive film is obtained by applying the conductive paste according to any of the foregoing onto the glass substrate, and making the paste sintered.
In addition, it is preferable that, in the glass article according to the present invention, a ceramic layer is formed on a surface of the glass substrate, and the conductive film is at least partially formed on a surface of the ceramic layer.